Radio frequency power sources generally should be impedance matched to load (or antenna) impedance for optimum power transfer to the load. Failure to match source and load impedances can not only result in waste of energy, but can result in excessive heating and possible damage to components in the source or damage to the load. Impedance matching is often achieved by measuring a Voltage (or current) Standing Wave Ratio (VSWR) at a point in a transmission line between source and load. An impedance matching network is then adjusted to optimize the VSWR. Of note, a radio frequency power source may be disabled when the VSWR exceeds limits.
Referring for a moment to FIG. 1, a simplified schematic of a conventional power coupler well known in the art is presented. As can be seen in FIG. 1, cross-coupled transformers 100, 102 are used for the directional coupler. In this model, power is sensed in both the forward and reverse directions on the main sampled line. Samples of forward and reverse power are rectified and filtered to provide dc analogs of the detected signals. The dc analogs are then used to calculate power and VSWR.
Limitations of the directional coupler of FIG. 1 include the requirement to acquire two samples of power to calculate VSWR, as well as associated bandwidth limitations. Further, measuring both forward and reverse power requires directional power couplers that can be bulky, dissipate some power, and can be difficult to optimize across wide frequency ranges. These directional power couplers may introduce nonlinearities which can produce undesirable harmonics. Many methods disclosed above produce significant distortion due to the use of certain transformers.
Hence, there is a need for an apparatus and method for determining power and VSWR in the transmission line of a RF source that overcomes one or more of the drawbacks identified above.